1. Field of the Invention
The present invention relates to slide fasteners, and more particularly to a slide fastener stringer having a continuous strip of thermoplastic molded coupling elements attached to a stringer tape along a longitudinal edge thereof.
2. Description of the Prior Art
Various slide fastener stringers are known in which a continuous strip of thermoplastic molded coupling elements interconnected by a pair of embedded connector threads is attached to a stringer tape along a longitudinal edge thereof. The molded coupling elements are continuously made on a die wheel having in its periphery an endless series of transverse cavities. The die wheel also has in the periphery a pair of annular grooves for receiving the connector threads intersecting the transverse cavities on opposite sides thereof. After having been bent into a U-shaped cross section, the molded coupling elements are attached to the tape by sewing stitches. However, since the only connection between the coupling elements is the connector threads, which are too flexible, only unstable attachment of the coupling elements to the tape can be achieved. This unstable attachment produces inferior slide fastener stringers with non-uniform spaces between the coupling elements. Further, it is difficult to cut such coupling element strip of a continuous length into a slide fastener length easily and precisely in the absence of a cutter of very high precision, in part because the connector threads are twisted yarns and are hence too flexible, and in part because practically a high degree of tension cannot be applied to the coupling element strip to keep the latter positionally stable during the cutting. This prior art is exemplified by U.S. Pat. No. 3,414,948.
To this end, one solution has been proposed in U.S. Pat. No. 4,290,175 (FIGS. 16 and 17) and Japanese Utility Model Post-Examination Publication No. 42-14900, for example, in which the successive coupling elements are interconnected by a pair of series of thermoplastic molded connecting portions disposed at opposite sides of the strip, each pair of the connecting portions extending between each adjacent pair of the coupling elements integrally therewith. Further, at each side of the strip, at least one connector thread is embedded in the succesive coupling elements and each series of the connecting portions and extends therethrough along the entire length of the strip. While this arrangement can solve the previous problem, a new problem is created in that an adequate degree of flexibility of the slide fastener cannot be achieved.